Organ transplantation is a very useful therapeutic method. Organ transplantation is classified into allotransplantation and xenotransplantation, which have their own merits and demerits. The human-to-human allotransplantation is an established method, but it is limited in the number of donors. Xenotransplantation from nonhuman mammal (for example, swine) to human is multiple in selection of grafts, but it is always accompanied by specific rejections such as hypoacute rejection (HAR) and acute vascular rejection (AVR).
To overcome such specific rejections of xenotransplantation, various methods have been proposed, such as a method of expressing human complement inhibitor in nonhuman mammals (for example, patent document 1), a method of decreasing Galα1,3Gal sequences (hereinafter referred to as α-Gal antigen) of non-reducing terminals of sugar chains not existing in primates or humans but existing in nonhuman mammals (for example, patent document 2), and a method of knocking out genes of α1,3 galactosyl transferase responsible for generation of α-Gal antigen (for example, non-patent documents 1 and 2).
[Patent document 1] Japanese unexamined patent publication No. H11-239430
[Patent document 2] Japanese unexamined patent publication No. 2002-291372
[Non-patent document 1] Science 2002, 295, 1089
[Non-patent document 2] Nat. Biotechnol. 2002, 20, 251
When a graft of nonhuman mammal is transplanted on human recipient, if the HAR can be overcome, the latter's antibody (anti-α-Gal antibody, etc.), complement, platelet, or natural killer (NK) cell may adhere to the former's cell, and the cell may be activated. The activated cell releases various cytokines, dissociates heparin, produces gaps against adjacent cells, exposes collagen of basement membrane, induces blood clotting reaction, closes blood vessels, and necrotizes grafts of nonhuman mammal (non-patent document 3). Such rejections are known as acute vascular rejections (AVR), and so far no method has been known to suppress efficiently the cytotoxicity of NK cell known as one of AVR factors.
[Non-patent document 3] Xenotransplantation 1998, 5, 169
NK cell adheres target cells by way of two types of receptors. They are killer cell activating receptor for inducing cytotoxicity, and killer cell suppressing receptor for suppressing cytotoxicity by recognizing the own MHC class I molecule. When the signal from the former surpasses the signal from the latter, the target cell is necrotized, but when the signal from the latter surpasses the signal from the former, the target cell is not necrotized.
The human cell expresses HLA class I molecules (HLA-A, -B, -C, -E, -F, -G), and hence the human cell is not damaged by human NK cell. On the other hand, sine the nonhuman mammal cell does not express HLA class I molecules, it is damaged by human NK cell. Hence, a new method has been developed to avoid cell damage by human NK cell by transforming the nonhuman mammal cell by genes of human HLA-A, HLA-B, HLA-C, or HLA-G (patent document 3). However, HLA-A, HLA-B, HLA-C are polymorphic, having 175, 344, and 90 alleles respectively, and it is not practical to prepare nonhuman mammal cells applicable to each HLA.
On the other hand, HLA-E and HLA-G are not polymorphic, and it has been attempted to use HLA-E and HLA-G. As a result, it is relatively easy to express HLA-G on the surface of nonhuman mammal cell, but suppression of cytotoxicity by human NK cell is low, and to the contrary it is found not easy to express HLA-E on the surface of nonhuman mammal cell, but suppression of cytotoxicity by human NK cell is high (non-patent document 4).
For the purpose of increasing the HLA-E expressing amount on nonhuman mammal cell surface, it has been attempted to use base sequence for coding the HLA-E, β2 microglobulin and HLA-A2 leader peptide (Val-Met-Ala-Pro-Arg-Thr-Leu-Val-Leu) (SEQ ID NO: 91), or base sequence for coding the leader peptide of HLA-G (Val-Met-Ala-Pro-Arg-Thr-Leu-Phe-Leu) (SEQ ID NO: 92). However, the HLA-E expressing amount by these transformants and the suppression of cytotoxicity by NK cell were not sufficient (non-patent document 5).
[Patent document 3] Japanese unexamined patent publication No. H11-510698
[Non-patent document 4] Transplantation Proceedings 2000, 32, 939
[Non-patent document 5] Transplantation 2002, 73, 1582